1. Field of the Invention
The present invention generally relates to a planar display structure, and more particularly to a planar display structure with LED (light emitting diode) light source.
2. Description of the Prior Art
Presently many display devices need to use plane light sources, such as transmissive-type/reflective-type liquid crystal display (LCD) or scanner so that the development of plane light source art is fast and attentive. The light source mainly used by LCD device is constituted by line light source formed with LED or cold cathode fluorescent lamp (CCFL). The light rays emitted from the light source at a lateral end of wedge light guide plate are guided almost vertically into the panel by the wedge light plate. The above structure is referred to U.S. Pat. No. 6,309,080, surface light source device and liquid crystal display. It has a main disadvantage that the light rays emitted from plane light source are not well distributed, in other words, the illumination of the portion of wedge light guide plate near the light source is higher than the other portion. Besides, the unsteady matter of CCFL that shortens the lift cycle of device is a drawback, too.
A method to solve the above problems is referred to US patent publication number 20020047837, which uses a array constituted by EL (electro-luminescent) components to replace plane light source that guides vertically the light rays into the panel by-light guide plate.
LED is a lighting component with low electricity and high illumination, and it has the steady lighting matter and long life cycle, too. It is very suitable for plane light source with LED as light source. Please refer to FIG. 1, a conventional structure of plane light source with LED light source is shown. The array constituted by LED 102 on a substrate 100 is used as plane light source. A light guide plate 104 is placed above the LED array 102 so that the light rays emitted from LEDs 102 can be well distributed. There are sequentially a first diffusor sheet 106, a prism sheet 108, and a second diffusor sheet 110 on the light guide plate 104 for the light rays emitted from the emitting surface of light guide plate 104 can be further well distributed. The above structure of plane light source is used as a back light source of LCD panel 120.
However, the aforementioned structure of plane light source has many drawbacks. First, the light rays emitted into the light-receiving surface of light guide plate 104 are not distributed. As shown in FIG. 1, the illumination of the section A of light guide plate 104 is darker than the section over LED 102 because the illuminating area of each LED is finite. A method that shortens the distance between two LEDs to decrease the area of the section A of light guide plate 104 can solve this problem. When the distance B in FIG. 1 is shortened, in other words, it will need the additional LEDs in the condition of same illuminated area. So this method is helpless to reduce manufacturing cost.
It is another method can solve the above problem to enlarge the thickness of light guide plate. As shown in FIG. 2, the area of the section A of light guide plate 105 is zero while the thickness of light guide plate is increased to a predetermined value. However, it is disadvantageous for minimizing volume and diminishing weight of electric devices. Besides, the manufacturing cost of light guide plate 105 is increased, too.
The utilization of light rays is not best in the conventional structure of light source because some of the light rays emitted from LEDs are reflected by light guide plate when the light rays enter into light guide plate. As shown in FIG. 1 and FIG. 2, the light rays C emitted from LEDs 102 will be reflected by the light guide plate 104 and 105 so that the utilization of light rays is worse. The above phenomenon still exists when the additional LEDs is used or the thickness of light guide plate is enlarged.